1. Field of the Invention
The present invention relates to a filter having resonators, the resonant frequency of which is adjusted with a dielectric layer formed on the resonators, and to a method of adjusting the resonant frequency of such resonators.
2. Description of Related Art
A filter characteristic such as resonant frequency has to be adjusted after its manufacturing process is completed, because the characteristic usually deviates from its target due to various deviation factors such as dielectric constant of a substrate, thickness of layers, accuracy of a mask, manufacturing process conditions and the like. Such characteristic adjustment, or tuning is especially necessary for narrow band and low ripple filters.
Conventionally, a resonant frequency of a filter has been tuned by adjusting an effective dielectric constant of a resonator with a screw that carries a dielectric member at its tip, or by trimming a resonator pattern with a laser beam. In both methods, it is necessary to carry out the adjustment or tuning while the filter characteristic is being measured. Such adjustment under measurement is not easy, especially when a filter is constituted by a large number of resonators.